Conventional terrestrial solar cells are generally made of thin wafers of silicon (Si) in which a rectifying or p-n junction has been created and electrode contacts, that are electrically conductive, have been subsequently formed on both sides of the wafer. A solar cell structure with a p-type silicon base has a positive electrode contact on the base or backside and a negative electrode contact on the n-type silicon or emitter that is the front-side or sun-illuminated side of the cell. The “emitter” is a layer of silicon that is doped in order to create the rectifying or p-n junction and is thin in comparison to the p-type silicon base. It is well-known that radiation of an appropriate wavelength incident on a p-n junction of a semiconductor body serves as a source of external energy to generate hole-electron pairs in that body. Because of the potential difference which exists at a p-n junction, holes and electrons move across the junction in opposite directions. The electrons move to the negative electrode contact, and the holes move to positive electrode contact, thereby giving rise to the flow of an electric current that is capable of delivering power to an external circuit. The electrode contacts to the solar cell are important to the performance of the cell. A high resistance silicon/electrode contact interface will impede the transfer of current from the cell to the external electrodes and therefore, reduce efficiency.
Process flow in mass production of electric power-generating solar cells is generally aimed at achieving maximum simplification and minimizing manufacturing costs. Electrode contacts in particular, are generally made by screen printing a paste containing metal and a glass frit.
A silver paste for the front electrode is screen printed then dried over the silicon nitride film. In addition, backside silver or silver/aluminum paste and an aluminum paste are then screen printed and successively dried on the backside of the substrate. Co-firing of front and backside pastes is then carried out in an infrared furnace at a temperature range of approximately 700° C. to 975° C. in air for a period of from several minutes to several tens of minutes.
During the co-firing, the front electrode-forming silver paste sinters and penetrates through the silicon nitride film during firing and is thereby able to electrically contact the n-type layer. This type of process is generally called “fire through” or “etching” of the silicon nitride
Conventional front electrode silver pastes contain silver powder, an organic binder, a solvent, a glass frit and may contain various additives. The silver powder functions as the main electrode contact material and provides for low resistance. The glass frit may contain lead or other low melting point constituents to give a softening point of about 300 to 600° C. so that during firing, it becomes molten and functions as the “fire through” agent wherein the silicon nitride is penetrated to allow the silver to make contact to the n-type silicon. The glass frit also provides for adhesion of the sintered silver to the silicon. Additives may be used as additional dopants to modify the n-type conductivity.
There is an on-going effort to improve efficiency of industrial silicon solar cells. One key focus is the reduction of contact resistance of the front face electrode contact. It is generally accepted that the contact formation of conventional screen printed silver pastes to the front face of solar cells involves a complex series of interactions between the glass, silver, silicon nitride and silicon. The sequence and rates of reactions occurring during the firing process are factors in forming the contact between the silver paste and the silicon. The interface structure after firing consists of multiple phases: substrate silicon, silver-silicon islands, silver precipitates within the insulating glass layer, and bulk sintered silver. As a result, the contact mechanism is a mix of direct ohmic contact by the silver-silicon island and silver precipitates and tunneling through thin layers of the glass. The extent of each of these components of the structure depends on many factors such as the glass composition, the amount of glass in the composition and the temperature of firing. Compositions and firing profiles of the silver paste are optimized to maximize cell efficiency. However, the presence of glass at the metal-silicon interface inevitably results in a higher contact resistance than would be realized by a pure metal contact to silicon.
The issues of forming good contacts to bipolar silicon devices are well known. All metal semiconductor contacts have a potential barrier that makes the contact rectifying. The lower the barrier height, the better the contact to silicon. There are several variables that control the barrier height, including the work function of the metal, the crystalline or amorphous nature of the silicon-metal interface, and the extent to which the interface is associated with charge carrier traps that pin the semiconductor Fermi energy. For example, using the Shottky limiting case for predicting band line up, for n-type silicon, if the work function for the metal is greater than the work function of the silicon, the contact between the two is rectifying. However, if the work function for the metal is lower than that of the silicon, the contact is ohmic. A metal cannot have low or zero barrier height on both n-type and p-type semiconductors. A metal that has a low barrier height on n-type silicon will have a high barrier height on p-type silicon and vice versa. Thus electrical contacts to silicon are optimized for the type of silicon. Low Shottky barrier height silicide contacts to n-type silicon semiconductor devices are well known. U.S. Pat. Nos. 3,381,182, 3,968,272 and 4,394,673, for example, disclose various suicides that form low Shottky barrier height contacts to bipolar silicon devices when the metal is placed in contact with the silicon and heated. However, such an approach has not been previously feasible with silicon solar cells due to the silicon nitride anti-reflective coating being a barrier to the reaction of the metal with the silicon.
The present inventors have created a novel process for making multi-element metal powders to be used to form front electrode contacts to silicon solar cells that eliminate the presence of a glass interface, which provides superior contact resistance and maintains adhesion.